1. Field of the Invention
This invention relates to an apparatus for joining optical fiber waveguides; and more particularly, to a system for adaptively positioning and orienting polarization-maintaining fibers for fusion splicing, so that the transmission loss of the joined fiber is minimized and the mode integrity is maintained.
2. Description of the Prior Art
Transmission of data by optical fiber waveguides, also called fiber optics or optical fibers, has become ubiquitous in the telecommunications and computer industries. Digital information in an electronic system is converted into a series of pulses of light generated by lasers or light emitting diodes (LED's), which are injected into long fibers of glass or polymeric materials. The fibers are capable of propagating the light with extremely low losses and acceptably low dispersion, whereby information embodied in the modulation pattern may be conveyed. The light that emerges from the other end of the fiber can be detected and reconverted into electronic signals that faithfully reproduce the original signal.
Fiber optic communication has a number of advantages over traditional transmission means such as hard-wired coaxial and twisted pair cable and lower frequency electromagnetic broadcasting such as radio and microwave. Foremost is the much larger bandwidth available. In addition, existing infrastructure such as cable ducts, utility poles, and the like presently used by telecommunications companies can be upgraded with relatively little disruption and moderate cost by substituting optical fiber cable for existing copper wire. Thus, dramatic increases in bandwidth needed to accommodate the needs of an information-based, Internet-driven society and commerce can be obtained with comparatively little disruption.
The bandwidth of a given optical communications system is further increased by use of polarization-maintaining (PM), single mode optical fiber. Such a PM fiber is characterized by some form of azimuthal asymmetry that results in very different propagation constants modes of two orthogonal polarizations. Cross coupling of the modes is very low, typically at a level of −20 to −30 dB.
Implementation of fiber optic systems requires both the equipment for actual transmission and processing of the data, and the equipment needed to install and maintain the fiber optic system and its infrastructure. The transmission and processing equipment, such as the fiber itself and the corresponding components needed to generate, detect, and process optically-borne information, have been developed to an ever increasing level of sophistication. While certain systems for joining and splicing fiber optic cables have been developed, there remains a need in the art for improved equipment and methods for splicing that are reliable, economical, and which result in minimal loss of signal integrity and strength, especially for polarization-maintaining fibers. Such systems, equipment, and methods are essential if the full inherent advantages of optical transmission are to be more widely implemented.
Together, these considerations call for splicing systems that are compact, portable, and able to be operated rapidly and reliably under adverse working conditions and with minimal slack cable. Moreover, it is desired that such a splicing system be capable of joining two fibers in a way that (i) causes minimal disruption or discontinuity in the optical transmission, (ii) does not adversely increase the diameter and volume of the cable, and (iii) has a durability as close as possible to that of an original fiber. Systems are also desired that are simple and reliable enough to be used by technicians who lack extensive training. There remains an urgent need for optical splicing equipment satisfying these requirements.
Optical fiber waveguides in common use share a number of structural features. The waveguide almost invariably comprises a thin, elongated fiber core responsible for conducting the light and at least one additional layer. Most often the fiber core is highly pure glass surrounded by a first and intimately bonded layer termed a cladding and an outer layer called a buffer. The cladding, usually also glass, has an index of refraction lower than that of the core to insure that light is constrained for transmission within the core by total internal reflection. Typically the buffer is composed of plastic or polymer and serves to protect the inner layers mechanically and to prevent attack by moisture or other substances present in the fiber's environment. Commonly a plurality of individual fibers (in some cases as many as a thousand) constructed in this fashion are bundled together and enclosed in a protective jacket to form a cable.
Commonly used fibers may further be classified as multimode or single mode. Multimode fibers typically comprise cores having diameters of 50–62.5 μm but in some cases up to 100 μm. Single mode fibers generally have a much smaller core that may be 9 μm or less in diameter. The glass-cladding diameter is most commonly 125 μm but sometimes is 140 μm (with a 100 μm core). The exterior diameter is largely a function of the buffer coating, with 250 μm most common, although some fiber coatings may be as much as 900 μm in diameter. Alignment of fibers is a crucial part of the preparation for any splicing operation, but is especially challenging for single mode fibers that have small core diameter. In order to produce a high quality, low-loss splice, the two opposing ends to be joined must be aligned laterally to within a small fraction of the core diameter. Of course, the smaller the fiber diameter, the smaller the allowed deviation from perfect abutting alignment that may be tolerated.
Most fiber optic data transmission systems transmit information using electromagnetic radiation in the infrared band, including wavelengths such as 850 nm for multimode fibers and 1310 and 1550 nm for single mode fibers. The nomenclature “light” is invariably employed for this radiation, even though the cited wavelengths fall outside the range visible to humans.
Two general approaches for splicing optical fibers are in widespread use, viz. mechanical and fusion splicing. Mechanical splicing is accomplished by securing the ends of two fibers in intimate proximity with an aligning and holding structure. Often the fibers are inserted into the opposing ends of a precision ferrule, capillary tube, or comparable alignment structure. The fibers are then secured mechanically by crimping, clamping, or similar fastening. An adhesive is also commonly used. In some cases a transparent material such as a gel having an index of refraction similar to that of the fiber cores is used to bridge the gap between the fibers to minimize reflection losses associated with the splice. Mechanical splicing is conceptually simple, and minimal apparatus is required to effect splicing. However, even in the best case, a mechanical splice has relatively high and undesirable insertion loss, typically 0.20 dB. In addition, mechanical splices are generally weaker than the underlying fiber and are notoriously vulnerable to degradation of the optical quality of the splice over time, especially under adverse environmental conditions such as varying temperatures and high humidity. Mechanical splices are generally regarded as being temporary expedients at best and are not useful for high bandwidth systems or permanent joints.
Fusion splicing entails the welding of the two fiber ends to each other. That is, the ends are softened and brought into intimate contact. The softening is typically induced by a small electric arc struck between miniature pointed electrodes mounted in opposition and substantially perpendicular to the common axis of the fibers. Upon cooling, a strong, low-loss joint is formed. When properly carried out, fusion splices exhibit very low losses along with high stability and durability rivaling those of the uncut fiber. Mechanical protection is often provided by a heat-shrinkable tube applied over the completed joint. The tube replaces the buffer coating that generally must be removed prior to splicing. In many cases the heat-shrinkable tube is reinforced by incorporation therein of a length of metallic wire for stiffness.
As noted above, careful preparation and precise lateral alignment of the ends of the fibers being joined is essential for forming low loss splices in both ordinary and polarization maintaining optical fibers. The axes of the fibers must be collinear within about 0.1 degree and aligned laterally within a small fraction of the core diameter to achieve the desired loss of less than about 0.03 dB. This required precision of alignment presents a substantial technical challenge, especially with single-mode fibers having cores approximately 9 μm diameter. Three general approaches have been proposed in the prior art. The simplest expedient is the use of mechanical fixturing, such as the alignment ferrules described above and other forms of pre-aligned V-grooves and the like. These purely mechanical approaches do not reliably produce splices that maintain less than 0.10 dB loss and so are ill suited for the demands of advanced, high-bandwidth communications systems. More sophisticated approaches employ some form of optically assisted fiber positioning. One such method is termed a profile alignment system (PAS). In this approach, the splicing apparatus incorporates an optical system that acquires images of the two fibers taken in two lateral directions, allowing the fibers to be positioned in two directions orthogonal to the mutual fiber axes. PAS systems may incorporate either manual positioning or may employ computerized image processing to optimize the alignment. However, the diffraction limit and pixel size of available electro-optic detectors restricts the precision achievable with PAS, even in systems based on visible light with wavelengths of about 400–700 nm. This particularly compromises the effectiveness of PAS in aligning small diameter, single mode fibers.
Still more advanced positioning methods have been proposed that employ measurement of actual light transmission between the fibers being joined. The positioning of the fibers is adaptively adjusted to maximize light transmission prior to the fusion operation. It is found that under carefully controlled laboratory conditions this approach may permit alignment better than that achievable with PAS systems.
The need for improved methods is especially acute for joining polarization-maintaining fibers. Whereas ordinary single mode fibers must be aligned with each other laterally and longitudinally to within about 1 micron, and in angle to within a fraction of a degree, polarization-maintaining fibers must also be aligned azimuthally; that is, they must be rotated relative to each other about their common axis until the fast and slow axes in their respective cores are also aligned. This is because it is essential for the successful application of these fibers that the transmitted light remain in the preferred polarized mode—either fast or slow—in crossing the splice. If the mode alignment is off by more than about 1 degree very serious losses and dispersion occur. Not only does the projected power become divided between the two orthogonal modes of the receiving fiber, but the coupling into the originally-excited mode is very poor. The quality of such splices is often characterized by measured values of extinction ratio, polarization extinction ratio (PER), or crosstalk. An effective means of aligning such fibers is thus clearly desirable.
In U.S. Pat. No. 4,612,028, there is disclosed a polarization-preserving single mode fiber coupler made without mutually aligning the polarization axes of the fibers by twisting the fibers together over a selected length and fusing them. A critical requirement of this coupling method is that the initial misalignment be not close to 90 degrees.
As taught in U.S. Pat. Nos. 5,156,663 and 4,911,524, the principal manner of aligning polarization maintaining single mode fibers has heretofore been to rotate a first fiber relative to a second fiber while exciting the first fiber and monitoring the output from the second. That is, the first “transmitting” fiber must be aligned with a polarized light source for injection of light aligned with the preferred axis. Likewise, the output end of the “receiving” fiber must have its preferred axis aligned with a polarizing filter and detector. Thereafter, the ends of the fibers to be spliced or coupled are brought together in a suitable stage or housing, for instance on a fusion splicer. After the ends have been aligned laterally with each other in x, y, and z dimensions, to maximize the coupling of power across the gap, one fiber is rotated slowly relative to the other while the power received at the photodetector is monitored. Eventually an orientation is found at which the coupling of power into the preferred axis is optimum. The fusion or mechanical splice is then completed, by fixing the oriented ends together permanently.
U.S. Pat. No. 5,244,977 to Anjan, et al. discloses a fiber optic polarization apparatus for use in the fabrication of fused optical couplers. U.S. Pat. No. 5,013,345 to Itoh, et al. discloses a method for fusion splicing of polarization maintaining optical fibers, while U.S. Pat. No. 5,149,350 to Itoh, et al. discloses an apparatus for fusion splicing of optical fibers.
U.S. Pat. No. 4,669,814 to Dyott discloses an optical fiber comprising a core and cladding having different refractive indices and forming a single-mode guiding region, where the core has a noncircular cross-section defining two refractive indices. Like the Anjan, et al. system, the Dyott system discloses a fiber in which light is injected along its length. The injection of light in the Dyott system is accomplished by a beam splitter.
Coupling of fibers using the methods described hereinabove require light to be injected along the length of the joining fibers. Rotational alignment is, alternatively, accomplished by the following methods: (1) coupled power monitoring, which is difficult and time consuming, and requires expensive input and output source and detector alignments; (2) axial imaging, which requires the fiber to have obvious and distinctive features; and (3) lateral imaging, in which the fiber must have obvious internal features amenable to a precisely-alignable image. With the first (power injection/detection) fiber alignment method, the set-up required to power and monitor the fibers is difficult and time-consuming to establish. Highly-skilled personnel are required; and the splicing procedure is itself time-consuming. If more than one pair of fibers is to be spliced, the process time and procedural difficulty increase dramatically. Methods (2) and (3) depend on imaging distinctive internal physical features of the fibers. If alignment is to be automatic, the system therefore requires sophisticated, expensive, and delicate image processing technology. If it is to be manual, the ability of a user to visually match two low-contrast images is oftentimes not accurate enough to yield rotational alignments of the required precision of 1 degree or better. Furthermore, few PM fibers exhibit images with distinguishable features, either in the axial or lateral views. Thus methods (2) and (3) are both difficult to implement and limited in applicability to a small proportion of the available PM fibers.
A system for splicing polarization-maintaining single mode optical fibers is disclosed by U.S. Pat. No. 6,203,214 to Wesson. The fibers are transversely illuminated and asymmetric stress in the fiber tip is measured using the photoelastic effect during rotation of the fibers about their long axes. The fibers are then rotated around their longitudinal axis to align their respective polarization axes and then joined together to produce a single polarization-maintaining optical fiber. The contents of U.S. Pat. No. 6,203,214 are incorporated herein in the entirety by reference thereto.
Notwithstanding numerous advances in the field of fiber optic joining, there remains a need in the art for an economical, efficient process for forming low-loss, durable, and reliable splices in polarization-maintaining fiber optic cables.